shopify-functions

Shopify Functions (Serverless Wasm Extensions)

Before writing code

Fetch live docs:

1. Fetch https://shopify.dev/docs/apps/build/functions for Functions overview
2. Web-search site:shopify.dev shopify functions input output for I/O schemas
3. Web-search site:shopify.dev shopify functions api reference for function types

What Are Shopify Functions

Serverless extensions that run on Shopify's infrastructure:

• Written in JavaScript or Rust
• Compiled to WebAssembly (Wasm)
• Execute within Shopify's checkout and backend pipeline
• 11 million instruction limit and 5ms execution time limit — must be extremely fast
• No network access, no filesystem — pure computation on provided input
• Wasm binary size limit: 256 KB, input JSON size limit: 64 KB

Function Types

Type	Purpose	Example
Discount	Custom discount logic	Buy X get Y, tiered discounts
Delivery customization	Modify shipping options	Rename, reorder, hide methods
Payment customization	Modify payment methods	Hide, reorder, rename gateways
Cart validation	Block or warn on cart conditions	Quantity limits, product combos
Cart transform	Modify cart contents	Bundle expansion, auto-add items
Fulfillment constraints	Control fulfillment behavior	Location priority, restrictions
Order routing	Direct orders to locations	Closest warehouse, priority

Input/Output Model

Functions receive structured JSON input and return structured JSON output:

Input (simplified)

{
  "cart": {
    "lines": [
      {
        "id": "gid://shopify/CartLine/1",
        "quantity": 2,
        "merchandise": {
          "__typename": "ProductVariant",
          "id": "gid://shopify/ProductVariant/123",
          "product": {
            "id": "gid://shopify/Product/456",
            "hasAnyTag": true
          }
        },
        "cost": {
          "amountPerQuantity": { "amount": "29.99", "currencyCode": "USD" }
        }
      }
    ]
  },
  "discountNode": {
    "metafield": {
      "value": "{\"percentage\": 10}"
    }
  }
}

Output (discount example)

{
  "discounts": [
    {
      "value": { "percentage": { "value": "10.0" } },
      "targets": [
        { "productVariant": { "id": "gid://shopify/ProductVariant/123" } }
      ],
      "message": "10% loyalty discount"
    }
  ],
  "discountApplicationStrategy": "FIRST"
}

Project Structure

extensions/my-discount/
├── src/
│   └── run.js          # Function entry point
├── input.graphql       # Defines what data the function receives
├── shopify.extension.toml  # Extension configuration
└── package.json

Configuration (shopify.extension.toml)

api_version = "2025-01"

[[extensions]]
name = "My Discount"
handle = "my-discount"
type = "function"

  [extensions.build]
  command = "npm exec -- shopify app function build"
  path = "dist/function.wasm"

  [extensions.ui]
  handle = "my-discount-ui"

  [extensions.input.variables]
  namespace = "my-app"
  key = "discount-config"

Input Query (input.graphql)

Defines what Shopify data the function receives:

query Input {
  cart {
    lines {
      id
      quantity
      merchandise {
        ... on ProductVariant {
          id
          product {
            id
            hasAnyTag(tags: ["vip"])
          }
        }
      }
      cost {
        amountPerQuantity {
          amount
          currencyCode
        }
      }
    }
  }
  discountNode {
    metafield(namespace: "my-app", key: "discount-config") {
      value
    }
  }
}

JavaScript Example

// src/run.js
export function run(input) {
  const config = JSON.parse(input.discountNode.metafield?.value || '{}');
  const percentage = config.percentage || 0;

  const targets = input.cart.lines
    .filter(line => line.merchandise?.product?.hasAnyTag)
    .map(line => ({
      productVariant: { id: line.merchandise.id }
    }));

  if (targets.length === 0) {
    return { discounts: [], discountApplicationStrategy: "FIRST" };
  }

  return {
    discounts: [{
      value: { percentage: { value: String(percentage) } },
      targets,
      message: `${percentage}% VIP discount`,
    }],
    discountApplicationStrategy: "FIRST",
  };
}

Performance Constraints

• 11 million instruction limit — function fails with InstructionCountLimitExceededError if exceeded
• 5ms execution time limit — Wasm module is killed if it runs longer
• Input JSON size limit: 64 KB — large carts may hit this
• No async operations (no Promises, no setTimeout)
• No network calls (no fetch, no HTTP)
• No filesystem access
• No global state between invocations
• Wasm binary size limit: 256 KB
• Rust is more instruction-efficient than JavaScript — consider Rust for complex logic
• Pre-compute and store config in metafields

Best Practices

• Keep function logic simple — no complex algorithms
• Use metafields for configuration (read via input query)
• Test with shopify app function run locally
• Use TypeScript for type safety (compiled to JS then Wasm)
• Handle edge cases: empty carts, missing metafields, zero quantities
• Return early when no action is needed (return empty discounts array)
• Validate all input — metafield values may be malformed JSON

Fetch the Shopify Functions documentation for exact I/O schemas, supported function types, and API version requirements before implementing.